1. Field of the Invention
The invention concerns an arrangement to transfer magnetic resonance signals emitted by individual antennas of a magnetic resonance local coil.
2. Description of the Prior Art
In magnetic resonance apparatuses, magnetic resonance signals are acquired with the use of local coils. A local coil generally is composed of multiple individual antennas that are also designated as “loops”. Each loop is generally fashioned as a coil.
During the magnetic resonance examination (data acquisition), excited atomic nuclei induce respective voltages as magnetic resonance signals in the individual antennas of the local coil, which magnetic resonance signals are transferred to a receiver. This transfer generally ensues via wires.
From DE 3500456 A1, U.S. Pat. No. 4,680,549 A1 and DE 19751017 A1 it is known to extend an individual local coil with an oscillating circuit, wherein the local coil is designed as a passive, resonant field concentrator and coupled with a readout coil for signal transmission.
In the acquisition case, a sensitivity gain is advantageously achieved here that corresponds to a concentration of the magnetic field. It is also advantageous that the local coil can be of simple design because, among other things, it requires no power supply.
However, it is disadvantageous that the cited prior art cannot be used without further measures for a local coil that has a number of individual antennas or acquisition coils.
Such a local coil exhibits a number of modes of the magnetic resonance signals (corresponding to the number of individual antennas) that must be transmitted. The modes can be associated with various spatial frequencies. Higher signal modes thereby have field distributions with a shorter range and are strongly attenuated in transmission, with an associated transmission power efficiency decrease and a proportional increase of the noise contribution of the readout coil.
A related problem is that, due to a relatively strong coupling among lower modes, these tend to dominate in the acquisition profile. Primarily due to an imperfect geometry of the local coil (for example a weakly concentric birdcage), it is difficult to read out a short-wave mode without interference due to signal and noise from a low mode.
An additional disadvantage results from the conventional use and associated wiring of low-noise preamplifiers. A low-noise preamplifier that amplifies the signal acquired by the individual antenna is respectively provided for every individual antenna of the readout coil. The individual antennas of the local coil retroact on the readout coil used for signal transmission. The source impedance at which the associated preamplifiers are operated thereby changes. Moreover, inputs of the various preamplifiers are coupled among one another.
An existing, internal preamplifier noise is disadvantageously increased by these two effects.